1. Field of the Invention
The present invention relates to electron beam apparatuses such as an electron microscope for allowing the phase change distribution of an electron beam due to the magnetic-field structure of a sample to be quantitatively visualized with a high resolution by taking advantage of the electron beam.
2. Description of the Related Art
In recent years, high-performance implementation of magnetic materials has been under way at a tremendous rate. As a result, measurement techniques for visualizing the magnetic-domain structures at nanometer level have become absolutely necessary for the development of new raw materials. Of these measurement techniques, a methodology which allows implementation of the highest spatial resolution and quantitative evaluation is the interference electron microscope method such as the off-axis electron beam holography method.
Hereinafter, referring to FIG. 2, the explanation will be given below-concerning the principle of the off-axis electron beam holography method. An electron beam, which is extracted from an electron source 1 by applying a voltage to a first extraction electrode 2 and a second extraction electrode 3, is accelerated up to a predetermined velocity by an acceleration electrode 4. Moreover, an electron beam with a high parallelism is formed by using such components as a first condenser lens 5 and a second condenser lens 6, and then a sample 7 is irradiated with the high-parallelism electron beam. Next, a voltage is applied to an electron beam biprism 19 which is located between an objective lens 8 and an image-forming lens system 11. As a result of this voltage application, an electron beam which has passed through the sample 7 and an electron beam which has passed through the vacuum in the vicinity of the sample 7 are superposed on each other on an image surface of the objective lens 8. This superposition forms an interference fringe, i.e., a hologram 10. Furthermore, this hologram 10 is magnified by the image-forming lens system 11, and is image-formed on a fluorescent plate 13, then being inputted into a detector 14. An input image from the detector 14 is introduced into a CPU 16 via an A/D converter 15. Then, after being subjected to an appropriate image processing, the input image is outputted to a display apparatus 18. Here, operation conditions on the components, such as the electron source 1, the first extraction electrode 2, the second extraction electrode 3, the acceleration electrode 4, the first condenser lens 5, the second condenser lens 6, the objective lens 8, the electron beam biprism 19, and the image-forming lens system 11, are controlled from the CPU 16 via a D/A converter 17. This interference fringe, essentially, is a one which should become straight lines. However, this interference fringe is phase-modulated by a magnetic field inside or outside the sample 7, thereby being shifted from the straight lines. From this shift amount, it is possible to reproduce the phase change amount via image processing such as, e.g., Fourier transformation method.
In JP-A-2002-117800, as an application embodiment of the off-axis electron beam holography method, an interference electron microscope using an electron beam biprism is disclosed. In the interference electron microscope described in JP-A-2002-117800, optical path of the electron beam is divided using the apparatus called the biprism, thereby generating an interference fringe on the transmission electron image. The unit used as the electron beam biprism is a one which is equipped with electrically-conductive property by applying metal evaporation on the surface of a glass fiber which is 300 nm to 600 nm in diameter and about a few mm long. Although no concrete disclosure is made in JP-A-2002-117800, phase information on the electromagnetic field of the sample is calculated based on the interference fringe generated.
In J. Cumings, A. Zettl, and M. R. McCarthy; “Carbon Nanotube Electrostatic Biprism: Principle of Operation and Proof of Concept”. Microsc. Microanal. 10 (2004) 420424, in a process where the observation is made using a transmission electron microscope in a state where electric potential is applied to carbon nanotubes, a possibility is studied that the carbon nanotubes can be used as the electron beam biprism of the off-axis electron beam holography method, and experiments associated therewith are made.
Meanwhile, as a methodology for making the observation of a phase object in a simplified manner, the Lorentz electron microscope method (defocus method) has been in use from olden times. In the Lorentz electron microscope method, however, there have existed two problems, i.e., low resolution and lack of quantitative property. In recent years, this defocus method has been improved. As a result, the TIE (: Transport of Intensity Equation) method, i.e., a methodology for visualizing the phase quantitatively, is disclosed in. e.g., Teague, M. R.; “Deterministic Phase Retrieval: A Green's Function Solution”. J. Opt. Soc. Am. 73 (1983) 1434 to 1441. Attention is now focused on this TIE method as an alternative method for the interference electron microscope method. This methodology is applicable to optical microscopes, X-ray microscopes, and electron microscopes.
Also, in V. V. Volkov and Y. Zhu; “Lorentz phase microscopy of magnetic materials”. Ultramicroscopy 98 (2004) 271 to 281, a proposal is made concerning the MTIE (: Magnetic Transport of Intensity Equation) method which results from applying the above-described TIE method to magnetic materials. Then, this MTIE method is applied to the visualizing of in-plane components of lines of magnetic force within a magnetic thin film. In this way, from the viewpoint of the quantitative property, the alternative methods such as the TIE method are not comparable to the interference electron microscope method. It is expected from the viewpoint of simplicity and wideness of observation area, however, that these alternative methods will be used from now on in a complementary manner with the interference electron microscope method.